Electronic Article Surveillance (“EAS”) systems are designed to prevent unauthorized removal of an item from a controlled area. A typical EAS system may include a monitoring system and one or more security tags. The monitoring system may create an interrogation zone at an access point for the controlled area. A security tag may be fastened to an item, such as an article of clothing. If the tagged item enters the interrogation zone, an alarm may be triggered indicating unauthorized removal of the tagged item from the controlled area. A security tag is deactivated before a tagged item can leave the controlled area without triggering the alarm.
As is known in the art, security tags (also referred to as labels) for EAS systems can be constructed in any number of configurations. The desired configuration of the tag or label is often dictated by the nature of the article to be protected. For example, an EAS label may be enclosed in a rigid housing which can be secured to the monitored item, such as hard tags containing EAS labels which are commonly attached to clothing in retail stores. Some EAS hard tags typically include a plastic tag body which houses an EAS sensor and a locking mechanism including a pin or tack which passes through the item and is clamped to the tag body to secure the item and tag together. Generally, theses tags require a detacher unit to remove the tack from the tag body and allow the item to be separated from the tag. In some applications, a detacher unit may include a magnet assembly which applies a magnetic field to the tag body for releasing the tack.
FIG. 1 illustrates a prior art EAS tag 10 having a rigid, e.g., plastic, tag body 12 with a hollow internal chamber 14. The tag body 12 houses an EAS sensor 16 for triggering an alarm. The EAS tag 10 includes a tack 18 with an enlarged head 20. As shown, the tack 18 is securely held within the tag body 12 by a magnetic clamping mechanism 22. In order to remove the tack 18, the magnetic clamping mechanism 22 must be disengaged using a magnetic detacher. The plastic tag body 12 includes a substantially circular protrusion 24 of sufficient size to completely encase the tack 18 and magnetic clamping mechanism 22.
FIG. 2 illustrates one conventional magnetic detacher unit 26. The magnetic detacher unit 26 includes a base unit 28 having an indented detaching zone 30 designed to receive protrusion 24 of EAS tag 10 or another magnetic securing device. A high field strength magnet assembly 32, as shown in FIGS. 3 and 4, resides within the base unit 28 and is positioned proximate to the indented detaching zone 28 to present a magnetic field within the detaching zone 30 in order to disengage a magnetic clamping mechanism 22 from a tack 18 of the EAS tag 10, thereby allowing removal of EAS tag 10 or other magnetic securing device from the previously secured item.
As is shown in FIG. 4, a magnet assembly 32 for a magnetic EAS tag detacher is shown. The magnet assembly 32 includes a cylindrical core magnet 34 and an oppositely magnetized ring magnet 36 stacked on top of the cylindrical core magnet 34 in order to maximize the axial magnetic field in proximity of a cavity 38 of the ring magnet 36. In other words, the magnetization of the cylindrical core magnet 34, indicated by field lines 39a, is opposite the magnetization of the ring magnet 36, indicated by field lines 39b in the body of the ring magnet 36. However, as the magnetic field of the ring magnet 36 radiates from body of the ring, the orientation of the magnetic field is actually rotated 180° when the field passes through the cavity 38. Therefore, within the cavity 38, the effects of the magnetic fields produced by the ring magnet 36 and the core magnet 34 are additive, thereby increasing the resulting field strength inside the cavity 38. As discussed below, using this arrangement, the maximum field strength is not provided at particular or optimal location.
The high field strength magnet assembly 32 includes a cylindrical core magnet 34 and an oppositely magnetized ring magnet 36 stacked on top of the cylindrical core magnet 34 in order to maximize the axial magnetic field in proximity of a cavity 38 of the ring magnet 36. To permit the removal of the tack 18, the protrusion 24 of the EAS tag 10 or other magnetic securing device is inserted into the cavity 38 to take advantage of the strong field inside the ring magnet 36. The magnet assembly 32 provides a substantially vertical magnetic field in the cavity 38 sufficient to force the clamping mechanism 22 to disengage and allow removal of the tack 18 from the tag body 12.
Many different types of magnetic clamping mechanisms 22 are used in a variety of EAS tags and other magnetic securing devices. For example, one such clamping mechanism 22 is shown in FIGS. 5 and 6. In this example, the clamping mechanism 22 consists of a spring 40 used in combination with a clutch 42. The shaft 44 of the tack 18 is inserted into a hollow tube 46 which extends through the protrusion 24 of the plastic tag body 12. The shaft 44 is inscribed with one or more notches 48a, 48b, 48c (referenced collectively as notch 48) which receive the clutch 42 in a locked configuration, thereby preventing the tack 18 from being removed from the plastic tag body 12. When the EAS tag 10 is secured (See FIG. 5), the spring 40 is in an engaged position supporting the clutch 42 and preventing the clutch 42 from moving in a downward direction and disengaging from the notch 48. When the EAS tag 10 or other magnetic securing device is presented with the magnetic field of the magnetic detacher unit 26 (See FIG. 6), the clutch 42, is pulled down and away from the notch 48 and releases the tack 18.
Other magnetic clamping mechanisms 22 may use different locking devices, but the principle operation of the magnetic detacher unit 26 remains the same as described above. To disengage a particular EAS tag 10 or other magnetic securing device, the high field strength magnet assembly 32 must present the needed magnetic field strength at the exact location of the implemented clutch 40. Because the field strength of the magnet assembly 32 decreases quite rapidly as the distance away from the magnet assembly increases, much stronger magnets than needed are often used in constructing the magnetic detacher unit 26. Stronger magnets introduce additional cost into manufacturing the magnetic detacher unit 26.
Additionally, the security tags used in an EAS system are replaced over time due to theft, loss, or normal wear and tear. For example, a sales clerk may forget to remove the EAS tag 10 from a purchased item. The security tags designed to be used in conjunction with a specific EAS system having a particular magnetic detacher unit 26 may be replaced with cheaper, “knock-off” EAS tags often provided by sub-standard manufacturers. These “knock-off” tags may not meet the requirements of the EAS system, provide a risk of unauthorized removal and do not, necessarily, have the magnetic clamping mechanism 22 at the same position of the original manufacturer's EAS tag 10. Often these “knock-off” tags may be easily detached using a single magnet, essentially rendering the protection offered by the EAS system practically worthless.
Therefore, what is needed is a system and method for optimizing the field profile of a high strength magnetic detacher in order to achieve maximum field strength at particular location.